An optical measured variable of process automation is, for example, the turbidity of a medium. In the following, the posed problem will be explained without limitation based on this measured variable.
Optical turbidity determination rests on scattered light measurement from undissolved particles suspended in the medium to be measured. In the case of use of turbidity sensors in containments, thus, for example, in pipelines or flow-through cells, wall effects lead to corruption of measured values. The light reflected or scattered on the inner surface of the pipe wall is detected by the sensor and incorrectly interpreted as a turbidity signal. A technique for minimizing these wall effects in pipelines involves an optimized sensor design (see, for example, the as yet unpublished application German DE 10 2013 103 735).
In order to minimize the undesired scattering on the tube, or pipe, wall, black, non-reflecting surfaces or larger dimensions of the pipeline can be used. This is, however, due to cost- or space reasons, not always possible. Also, frequently, a certain material must be used, for instance, for hygienic applications.
Even with the above mentioned improvements, one cannot, as a rule, completely remove the influence of wall effects on the measured value, above all, in the case of small tube diameters, respectively small wall separations and/or reflecting materials.
Since the influence of the wall effects changes with the turbidity value (wall effects have, as a rule, a greater influence in the case of lesser cloudings; wall effects have, as a rule, lesser influence in the case of greater cloudings), it is for the user very difficult and very complex to eliminate these effects by means of calibration. The user would have to perform comparative measurements at a number of turbidity values, and these are often error susceptible, time intensive and expensive, or bring about a state, which, as a rule, cannot be synthetically generated. Thus, in real systems, turbidity values cannot be varied as much as desired.